1. Field of the Invention
The present invention relates to a dielectric porcelain composition for high frequency waves, used in a high frequency band of e.g., microwaves or millimeter waves, particularly, to a dielectric porcelain composition useful as a material for circuit boards of microwave integrated circuits, millimeter waves integrated circuits and the like, dielectric strips and dielectric antennas used in a microwave band and a millimeter wave band, and to a dielectric resonator and nonradiative dielectric strip using the same.
2. Description of the Related Art
There have been cases in high frequency circuits such as microwave integrated circuits and millimeter wave integrated circuits that such a structure is employed that a dielectric porcelain for resonance is fixed on a dielectric board through a dielectric supporting member.
FIG. 1 is a cross sectional view showing a constitutional example of a dielectric resonator. That is an example of a dielectric resonator applied to a dielectric resonator control type microwave oscillator, constituted such that a dielectric porcelain 1 is attached to a dielectric board 3 through a dielectric supporting member 2 and is electromagnetically coupled to a strip line 4 formed on the dielectric board 3 by utilizing an electromagnetic field H which leaks outside the dielectric porcelain 1, which are housed in a metallic container 5.
Since a resonance system of high unloaded Q level can be constituted in the high frequency circuit by controlling the leakage of electric field of the dielectric porcelain 1 through the dielectric supporting member 2, it is necessary to use, for the dielectric supporting member 2, a material having a low dielectric constant and a small dielectric loss (tanδ), i.e., a large Q value. Therefore, forsterite (2MgO·SiO2) ceramics having a dielectric constant of about 7 and a Q value at a measuring frequency of 10 GHz of about 15,000 has been used as the material for the dielectric supporting member 2, and alumina ceramics having a dielectric constant of about 10 and a Q value at a measuring frequency of 10 GHz of about 20,000 or more has been mainly used as the material for the dielectric board 3.
Cordierite (2MgO·2Al2O3·5SiO2) ceramics has been known as a dielectric material having a low dielectric constant. Since a dense sintered body of the cordierite ceramics is difficult to obtain owing to its remarkably narrow sintering temperature range, glass ceramics has been known that is obtained by adding a glass material to result a dielectric constant of from 4 to 6 and a Q value at a measuring frequency of 10 GHz of about 1,000.
Furthermore, there has been known a nonradiative dielectric guide (hereinafter referred to as an NRD guide) having a guide for transmitting a high frequency signal comprising a dielectric material.
FIG. 2 is a partially cutaway perspective view showing a basic constitution of an NRD guide of the invention and the conventional art. An NRD guide S1 comprises a dielectric strip 12 intervening between a pair of parallel flat conductive bodies 11 and 13 having a distance of λ/2 or less, in which λ is the wavelength of a high frequency signal (electromagnetic wave), such as a millimeter wave, propagating in the dielectric strip 12. In the NRD guide S1, the electromagnetic wave is shielded and cannot enter from the outside when the distance of the parallel flat conductive bodies 11 and 13 is ½ or less of the wavelength λ of the high frequency signal, but when the dielectric strip 12 is made intervene between the parallel flat conductive bodies 11 and 13, the electromagnetic wave can propagate along the dielectric strip 12 inside the same, and a radiation wave is suppressed by the shielding effect of the parallel flat conductive bodies 11 and 13. In FIG. 2, a part of the upper parallel flat conductive body 13 is cut for viewing the interior. The wavelength λ of the high frequency signal is that at the use frequency in the air.
The electromagnetic wave propagation mode of the NRD guide S1 includes two modes, i.e., the LSM (longitudinal section magnetic) mode and the LSE (longitudinal section electric) mode, and the LSM mode that exhibits a small loss is generally employed.
A curved dielectric strip 12 can also be used, and in this case, since an electromagnetic wave can easily be propagated in a curvilinear form, an advantage can be obtained in that a millimeter wave integrated circuit can be downsized, and high flexibility in circuit design can be obtained.
As the material of the dielectric strip 12 of the NRD guide S1, a resin material having a dielectric constant of from 2 to 4, such as Teflon and polystyrene, has been conventionally used owing to its easiness in processing.
However, the dielectric constants of alumina ceramics and forsterite ceramics used in the conventional resonator are about 10 and about 7, respectively, and therefore, a material having a lower dielectric constant is being demanded associated with spreading of a dielectric resonator for a high frequency band in recent years.
On the other hand, porcelain, such as glass ceramics, which is generally used as a low dielectric constant material, has a small dielectric constant of about from 4 to 6 but has a Q value of about 1,000 at 10 GHz, and therefore, a low dielectric constant material having a higher Q value is being demanded associated with spreading of a dielectric resonator for a high frequency band in recent years.
Furthermore, since alumina ceramics, which is mainly used as the dielectric board 3 of the dielectric resonator, has a relatively high dielectric constant of about 10, it involves such a problem that when a strip line of a high impedance is to be formed, the line width is too decreased to about 1 μm or less to cause breakage, and fluctuation in the relative line width is increased, whereby the defective fraction is increased when a microwave integrated circuit is fabricated by using the dielectric resonator.
This is because the impedance of the strip line in the dielectric board 3 is inversely proportional to the dielectric constant thereof and the width of the strip line, respectively, assuming that the thickness of the dielectric board 3 is constant. Therefore, the impedance can be increased by using a material having a lower dielectric constant instead of reduction in the width of the strip line. Thus, a material having a low dielectric constant is being demanded.
In order to solve the problems, the inventors have proposed a dielectric porcelain composition for a high frequency and a dielectric resonator, which comprise a complex oxide containing Mg, Al and Si as metallic elements, in which the molar composition of the respective metallic elements in the oxide xMgO·yAl2O3·zSiO2 satisfies 10≦x≦40, 10≦y≦40, 20≦z≦80, and x+y+z=100, the dielectric constant is 6 or less, and the Q value of 2,000 or more at a measuring frequency of 10 GHz (Japanese Unexamined Patent Publication JP-A 9-48661 (1997)).
The dielectric porcelain composition for high frequencies is of excellent properties as having a dielectric constant lower than those of alumina ceramics and forsterite ceramics, and a Q value higher than that of glass ceramics. However, a dielectric porcelain composition exhibiting a high Q value at a higher frequency is still demanded.
When the conventional NRD guide is constituted by a dielectric strip using a dielectric material comprising a resin material, such as Teflon and polystyrene, there is a problem that the curvature loss at a curved part of the dielectric strip and the loss at a junction of the dielectric strips are large. Therefore, a sharp curved part cannot be formed in the dielectric strip, which brings about, as a result, such a problem that the NRD guide has a large size. In the case where a loose curved part is formed in the dielectric strip, it is necessary that the curvature of the curved part be precisely determined to suppress the high frequency signal loss.
Furthermore, the frequency range that can be used under the condition that the curvature loss is small is 1 to 2 GHz in the vicinity of 60 GHz, which is insufficient. This is because in the case where the NRD guide is constituted with a dielectric material having a dielectric constant of from 2 to 4, the distance between the LSM mode and the LSE mode is too close as about 3 GHz, and thus a part of the electromagnetic wave in the LSM mode is converted to the LSE mode. That is, with respect to the diffusion characteristics of the LSM mode and the LSE mode, the diffusion curves of the two modes is separated from each other by only about 3 GHz at B/B0=0 (B represents a propagation constant of a high frequency signal in a dielectric strip, and B0 represents a propagation constant of a high frequency signal in vacuum), which causes the conversion of a part of the electromagnetic wave in the LSM mode to the LSE mode. There has been a product using ceramics having a dielectric constant of about 10, such as alumina, as the material of the dielectric strip, but in order to use it at a high frequency of 50 GHz of higher, it is necessary that the width of the dielectric strip is extremely narrow, and it is not practical on processability and workability of fabrication, i.e., on productivity.
Furthermore, the cross section of the dielectric strip becomes smaller when the frequency becomes higher. For example, in the case where a dielectric strip having a cross sectional size of about 1 mm×2 mm and a length of about 10 mm is formed with porcelain and arranged, a problem occurs in that the dielectric strip is extremely liable to broken on handling upon production. Moreover, it is necessary to retain the dielectric strip by a pair of parallel flat conductive bodies, but a problem occurs in that the dielectric strip is broken upon fastening with the parallel flat conductive bodies.